Various methods of fabricating resistors on a semiconductor substrate are known. U.S. Pat. Nos. 4,140,817, 5,548,268, 5,683,928, 5,976,392, 5,989,970, 6,069,398, and 6,093,596, each of which is hereby incorporated by reference, discloses a method of manufacturing resistors.
In an integrated circuit, a metal, such as a trace, passing over the body of a high sheet resistance diffused resistor, can cause variations in the resistance of the resistor when a voltage is applied to the trace. The voltage on the trace could cause a region of the resistor beneath the trace to invert, deplete, or accumulate, which would result in resistance variations in the resistor. Through repeated occurrences, undesirably, a permanent change to the resistance could occur.
One solution has been to not route metal conductors over resistors to obviate the problem. This technique, however, wastes valuable area and causes integrated circuit die employing this technique to be larger in area than integrated circuit die utilizing the area over resistors for routing metal conductors.
Another solution, illustrated in FIG. 18, has been to extend over the resistor body the metal trace that connects to a first one of the resistor contacts. The metal extension, known as a field plate, would extend almost to the metal that connects to the second resistor contact, as layout, design, and fabrication rules allow. In this manner, the voltage applied to the first resistor contact will also be applied to the field plate over the resistor body. The second resistor contact is connected to another potential. There remains variation in the resistance of the resistor due to voltages applied to the first contact and field plate, however, at least the voltage is known. A shortcoming of employing a metal field plate is that the area over the resistor body, excluding the contact areas, is not available for routing other metal conductors in the same layer of metal as contacts to the resistor. Of course, metal conductors could be routed over the resistor body in higher layers of metal, as is known in the art.
Yet another solution, illustrated in FIG. 19, has been to provide a polysilicon field plate over the body of the resistor. The metal trace that connects to a first one of the resistor contacts is extended to also contact the polysilicon field plate. Using this technique, a portion of the area over the body of the resistor is available for routing other metal conductors in the same layer of metal as contacts to the resistor. Since the metal that connects to the first one of the resistor contacts makes a second contact with the polysilicon field plate, the area of the contact with the polysilicon field plate, as well as any area near the contact with the polysilicon due to layout, design and fabrication rules, is not available for routing other metal conductors in the same layer of metal as contacts to the resistor.
What is needed is a field plate resistor that permits substantially all of the area over the body of the resistor that layout, design, and fabrication rules permit to be available for routing metal conductors in the same layer of metal as contacts to the resistor.